Active sliding generated by cross-bridges between filaments is believed to be the basis for movement in flagella and cilia, as well as in muscle. Computer simulation methods have been developed for relating the movement of a flagellum to properties of its active moment generating system. Preliminary work has been done with simple computer models which incorporate cross-bridge properties, but the specification of cross-bridge properties used in these models is not adequate to explain existing biophysical data on flagellar movement. Continued work in this direction is essential in order to take full advantage of new information from experimental work with flagella. This work should lead to new insights into cross-bridge function in flagella, which may be applicable to muscle and other motile systems. Additionally, this work is prerequisite to computer simulation analysis of the function of the outer dense fibers in mammalian spermatozoa, which could form a basis for development of more specific contraceptive agents.